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Here's the deal. T-Mobile is in the middle of three major wireless transitions. Besides ripping up mandatory two-year contracts, the carrier is building a new 4G LTE network. If that wasn't enough, T-Mobile also is migrating its older 3.5G data service from the AWS band (1700/2100) where it currently lives over to the 1,900MHz GSM band.

But that's not all. T-Mobile is using that freed-up AWS spectrum to build out its fledgling 4G LTE network. Coincidentally, or maybe not, that frequency shuffle places T-Mobile's airwaves in close alignment with AT&T's. So even without that failed merger, a big change is happening.

Have AT&T iPhone 5, will carry
So what does this mean? It means a great deal, especially if you're wondering whether you can easily port AT&T-branded handsets such as the iPhone 5 over to T-Mobile. I'm sure there are more than a few AT&T subscribers eyeing T-Mobile's cheaper and no-contract plans with interest. That goes double for AT&T iPhone 5 owners.

Sadly, though, there are a couple of technical factors getting in the way. First off, any iPhone 5 you purchase from AT&T with a subsidy will be locked to the carrier. That means you can't use it on T-Mobile at all. But let's say that you have an unlocked iPhone 5, as is possible if you paid full price for your handset or if you bought it from Apple directly. Will you then be good to go after you pop in a T-Mobile SIM? The answer is yes, but only to a point.

As Engadget reported yesterday, the current GSM iPhone 5 and the forthcoming handset from T-Mobile will have the same model name (A1428). That sounds great, but the problem is that the existing model doesn't support T-Mobile's AWS bands for top HSPA+ speeds.

Slow 3.5G
The result of that technicality is that while an existing unlocked iPhone 5 could connect to T-Mobile's HSPA+ networks, it can do so only at slower speeds. As T-Mobile confirmed with CNET, instead of connecting at the top speed of 42Mbps, you'd top out at a slower data speed of 21Mbps. Additional details from Apple also mention the tricky HSPA+ situation.

So what can you do? Well, if you want an iPhone 5 that supports the 1700 and 2100 AWS bands and will give you HSPA+ data speeds of up to 42Mbps, you'll have to wait for April 12. That's when Apple and T-Mobile will start selling such a handset.

At that point, Apple will have transitioned to producing just one GSM model (also A1428) that will deliver full support for AT&T's and T-Mobile's networks. Just remember that you may have to unlock an iPhone before you jump carriers.

Meanwhile, in the absence of a full-speed T-Mobile 4G HSPA+ signal you'll still be able to make calls, send messages, and transfer data on an unlocked AT&T iPhone 5. It's just that your surfing will go much slower than you would like.

Other hot and unlocked phones
But the iPhone isn't the only smartphone people care about -- far from it. If you're planning to buy a hot unlocked global smartphone like, say, the HTC One model to run on T-Mobile LTE, you're out of luck as well. The AWS spectrum for LTE is limited to the U.S., so unless the device you intend to carry over to T-Mobile supports LTE band 4 (AWS), you'll be stuck on 3G/3.5G speeds. Well, I guess that's better than EDGE.

Still, T-Mobile did assert that "most" unlocked phones connecting to LTE 4G in the AWS band will be compatible with its network. Of course with such low, no-contract prices, it might make more sense to scoop up a T-Mobile handset and call it a day. That is, if you can find a 4G LTE signal near you.

Update, Wednesday at 5:08 p.m. PT:Added more details from Apple.

Correction, Thursday at 11:30 a.m. PT:The 21Mbps limitation applies only to T-Mobile's HSPA+ network and not to its LTE network as originally reported.

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T-Mobile wows with fresh, fast 4G LTE phones

About the author

Brian Bennett is senior editor for appliances at CNET and reviews a wide range of household and smart-home products. These include everything from microwave ovens, blenders, ranges and coffee makers to personal weather stations. An NYC native, Brian now resides in bucolic Louisville, Kentucky where he dreams of someday owning the sparkling house of the future.
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